<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .container{
            border: 2px solid black;
            overflow: hidden;
        }

        #l, #r{
            width: 49%;
            height: 300px;
            background-color: yellow;

            border: 2px solid black;
        }

        #l{
            float: left;
        }

        #r{
            float: right;
        }

        /* 假设此处元素为 250px * 250px  */
        #l:before, #r:before {
            content: "";
            width: 125px;
            height: 250px;
            background: blue;
            border: 2px solid black;
        }

        #l:before {
            float: right;
        }

        #r:before {
            float: left;
        }

    </style>
</head>
<body>
<div class="container">
    <div class="float-center-box"></div>
    <div id="l"></div>
    <div id="r"></div>
</div>
</body>
</html>